Turbidity removal effect and surface charge shift for electrochemically treated retentate without coagulant addition Toshiya Uchibori, Takeshi Fujino and Takashi Asaeda ABSTRACT Toshiya Uchibori Takeshi Fujino (corresponding author) Takashi Asaeda Department of Environmental Science, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan E-mail: fujino@mail.saitama-u.ac.jp An electrolytic treatment method promoting dense aggregates was developed in order to thicken retentate quickly without coagulant addition. A kaolin suspension with a turbidity of 200 NTU with a large fraction of colloidal particles was used as the retentate. Comparative testing showed that the electrolytic treatment increased aggregate size and enhanced the turbidity removal effect up to 75% on average with increasing retention time. Even though the Al ion concentration in the treated retentate was much lower than 0.1 mg/L, along with the large upward shift of surface charge, the turbidity removal effect was enhanced considerably with independently stabilized pH compared with alum as the coagulant. Comparison between the charging behaviors indicated that the electrochemical treatment generates polymeric Al hydroxide species that form adsorption layers with fewer defects, thereby inducing a stronger removal effect. Key words | electrolytic treatment, polymeric Al hydroxide species, retentate, surface charge, turbidity removal effect INTRODUCTION In water or wastewater treatment systems, processes promoting the growth of particle aggregates play an important and basic role in pretreatment steps in solid – liquid separation processes such as coagulation and settling. These separation processes rely on forces between particles, especially in the colloidal size range. Inter- particle forces are almost linearly dependent on the particle size. These become stronger as the particle size decreases, relative to external forces (Gregory 1993). Effective turbidity removal from water by such processes is dependent on the degree of charge neutralization that strengthens van der Waals attraction between particles. As for conventional charge neutralization process, chemical coagulants such as sulphate, chloride have been used in order to produce multivalent cations in water. However, sludge produced with coagulant dosage increases its water- retention. However, sludge produced with coagulant dosage increases its water-retention ability resulting in a watery voluminous sludge with a low dewaterability, which may lead to a decrease in efficiency during the subsequent sludge treatment (Bruns et al. 1992). The membrane filtration process for raw water without coagulant dosage has been used increasingly in Japan (J.W.R.C. 2008) and has the advantages of a high solid – liquid separation ability and a reduced sludge volume. However, this process requires regular membrane washing for practical use to maintain its high separation ability and consequently yields highly concentrated wastewater as the retentate. The thickening of the retentate due to gravitational settling in a thickener requires a long retention time since the retentate has a large fraction of colloidal particles without coagulant dosage. Therefore, a microfilter thickener (Uchibori et al. 2008) has recently been studied to an alternative sludge treatment method without coagulant dosage. While, the present electrocoagulation (EC) method, in which metal ions are dissolved from an anode and used doi: 10.2166/wst.2010.479 235 Q IWA Publishing 2010 Water Science & Technology—WST | 61.1 | 2010 Downloaded from https://iwaponline.com/wst/article-pdf/61/1/235/447770/235.pdf by guest on 29 May 2020